Centrex service

In telecommunication, a Centrex service (CTX) is a service offered by Bell Operating Companies that provides functions and features comparable to those provided by a PBX or a PABX. Note: "Centrex C.O." indicates that all equipment except the attendant's position and station equipment is located in the central office. "Centrex C.U." indicates that all equipment, including the dial switching equipment, is located on the customer's premises.

Source: from Federal Standard 1037C